Theoretical and Experimental Study of the Spectroscopy and Thermochemistry of UC^+/0/–

Abstract

A combination of high-level ab initio calculations and anion photoelectron detachment (PD) measurements is reported for the UC, UC–, and UC+ molecules. To better compare the theoretical values with the experimental photoelectron spectrum (PES), a value of 1.493 eV for the adiabatic electron affinity (AEA) of UC was calculated at the Feller–Peterson–Dixon (FPD) level. The lowest vertical detachment energy (VDE) is predicted to be 1.500 eV compared to the experimental value of 1.487 ± 0.035 eV. A shoulder to lower energy in the experimental PD spectrum with the 355 nm laser can be assigned to a combination of low-lying excited states of UC– and excited vibrational states. The VDEs calculated for the low-lying excited electronic states of UC at the SO-CASPT2 level are consistent with the observed additional electron binding energies at 1.990, 2.112, 2.316, and 3.760 eV. Potential energy curves for the Ω states and the associated spectroscopic properties are also reported. Compared to UN and UN+, the bond dissociation energy (BDE) of UC (411.3 kJ/mol) is predicted to be considerably lower. The natural bond orbitals (NBO) calculations show that the UC0/+/– molecules have a bond order of 2.5 with their ground-state configuration arising from changes in the oxidation state of the U atom in terms of the 7s orbital occupation: UC (5f27s1), UC– (5f27s2), and UC+ (5f27s0). The behavior of the UN and UC sequence of molecules and anions differs from the corresponding sequences for UO and UF.

Bonding, Thermodynamics, and Spectroscopy of the Metal Borides UB0/+/– and WB0/+/–

“…We can compare the BDE(UB) to the experimental value of Gingerich 32 who obtained BDE(UB) = 318 ± 33 kJ/mol. If we use an improved value for BDE(UC) 25 for the UB + C → UC + B reaction, we obtain BDE(UB) = 268 ± 33 kJ/mol and our value of 223 kJ/mol is consistent with the lower range of the experimental value given the difficulties with interpreting such experimental data.…”

“…The calculated BDE for UB at the FPD level is 223.1 kJ/mol. Compared to the calculated BDE of 411.3 kJ/mol for UC and a calculated value of 561 kJ/mol for UN, the BDE­(UB) is considerably lower than those for UC and UN. For UB, the spin–orbit correction is −11 kJ/mol and the other corrections increase the BDE by only 3 kJ/mol.…”

“…Gingerich reported a BDE for UB based on a combination of Knudsen effusion and mass spectrometric techniques for the reaction UB­(g) + C­(g) = UC­(g) + B­(g). The reported BDE of 76 ± 8 kcal/mol has large error bars and is based on a value for BDE­(UC) with large error bars that is not consistent with more recent reliable values of this BDE …”

“…The reported BDE of 76 ± 8 kcal/mol has large error bars and is based on a value for BDE(UC) with large error bars that is not consistent with more recent reliable values of this BDE. 25 In the current work, we have extended our series of calculations on diatomic uranium main group diatomic molecules to the properties of UB +/0/− at the CASPT2-SO level. We have also calculated the electronic structure of the corresponding WB compounds to examine the role of d vs f orbitals on the metal atoms.…”

“…The diatomic molecules formed by the U atom with main group elements are being explored by a combination of experimental spectroscopy techniques and high-level computational electronic structure methods based on correlated molecular orbital theory at the coupled cluster and multireference configuration interaction levels. − The electronic structures of UC, UN, UO, and UF as well as their anions and cations have been studied in detail. The electronic structures of UC and UN and their anions and cations show that the changes in adding or removing an electron occur in the U 7s orbitals.…”